1. Field of the Invention
This invention relates to an integral beaded stylus wire for use in a matrix printhead, and a method of making same.
2. Prior Art
High speed printers capable of printing a character in milliseconds enjoy widespread use in computer applications. Since the limiting factor on the rate of computer output is the time required for the actual printing function, efforts are continually ongoing to develop a faster and more efficient means of high speed printing. A component of the high speed printer which is highly successful and widely used to perform the actual printing function is a matrix printhead.
The matrix printhead relies on a plurality of thin elongated wires to physically impress the ink onto the paper. These wires are commonly referred to as stylus wires. When a particular letter or character is to be printed, an appropriate set of these wires is thrust forward against an inked ribbon forcing the ribbon onto the paper. Each wire has an impact receiving end and a printing end which contacts the ribbon so that a minute dot is imprinted on the paper. The result is a pattern of dots formed on the paper in a configuration corresponding to the desired character.
In the past, the enlargement or head formed at one end of the wire consisted of a spiral or pig-tail like loop of wire covered in plastic. This wire was extremely weak at the joint where the pig-tail loop met the wire stem, requiring that the wire be handled with great care. These pig-tail type wires were very susceptible to high stress concentrations, fatigue, and eventual failure in the matrix printhead application. The common failure encountered was a breaking off of the pig-tail loop from the wire stem due to the inherent weakness of the connection joint under compression.
It is an object of the present invention to provide a printwire that is not inherently weak or brittle, but is strong and resilient so as not to break or crack even after a relatively large number of compressive applications. Preferably, the wire remains firm under compression while impressing the dot upon the paper, with the flexibility of the wire allowing it to bend slightly to relieve the stress. When the compressive force is relaxed, the resiliency of the wire enables it to spring back into its original shape.
Many of the stylus wires currently in use were formed from a metal having characteristics of strength and resiliency. But the forming techniques used to shape the pig-tail loop or similar enlargement on the end of the relatively long metal wire resulted in the wire head and connecting joint losing those desirable properties of the metal. In addition, these forming techniques proved to be cumbersome and inefficient in producing a consistent head shape in a mass production environment.
It is a further object of the present invention to provide a method of forming a reliable enlargement onto the thin metal wire without losing the advantageous physical properties or characteristics of the metal. It is also an objective that this method yield consistent wire head shapes at a high rate of production.